The present invention relates generally to Quality of Service (QoS) for a wireless network, and more particularly to a simplified Hybrid Coordination Function implementation for an 802.11 network.
Unless otherwise defined herein, the terms in this specification should be interpreted as defined, or as customarily used, in the Institute of Electrical and Electronics Engineers (IEEE) 802.11 and 802.11e specifications. The IEEE 802.11 and IEEE 802.11e specifications are hereby incorporated by reference. The current draft standard refers to the current draft supplement to the 802.11e specification, which is hereby incorporated by reference.
The current 802.11e draft standard for QoS defines two access methods—a Carrier Sense Multiple Access (CSMA)-based access method called EDCF and a hybrid access method call the Hybrid Coordination Function (HCF). In the HCF access method, a Hybrid Coordinator (HC) sends a poll to a client station (station) to grant the station a polled Transmission Opportunity (TXOP).
It is generally agreed that EDCF is not sufficient for QoS for interactive voice in an enterprise environment with “hidden” stations. However, HCF, as originally defined in the 802.11e draft, requires a very complex AP implementation. Therefore, most AP vendors (including Cisco) do not want to support it, at least not in an initial 802.11e-compliant release.
Thus there exist a need for an AP HCF implementation that is greatly simplified and more “client friendly”.
In addition to the aforementioned need, the error recovery mechanism, currently specified in the 802.11e draft, for HCF polled access is complicated and ambiguous. If the HC misses an expected response to a poll then both the HC and the station initiate error recovery. In some cases, the 802.11e error recovery mechanism can result in repeated collisions. Thus, the need exists for a simple method for handling polling error recovery.
In addition to the aforementioned needs, the current 802.11e draft describes two disparate QoS power-save methods. In an “Automatic Power-Save Delivery” method (APSD), stations establish “Wakeup Beacons” and the AP delivers buffered downlink frames immediately following the Wakeup Beacons.* An AP indicates that it has frames buffered for a station by setting the stations bit in the TIM in Beacons and Probe Response frames. In an HCF “Service Period” method, service periods for a station are separated by a minimum inter-service period. The AP can only transmit to a power-save station at the start of the station's service period, so that the station can “sleep” between service periods. The Service Period method has a known flaw where service period start times may become unsynchronized in the AP and station.
The APSD method is much simpler than the HCF “Service Period” power-save method described in the current 802.11e draft. APSD also has the advantage that, unlike the is Service Period method, it works the same with EDCF or HCF. However, the APSD method in the current 802.11e draft has several drawbacks compared to the “Service Period” method: a) APSD requires a fast Beacon rate to support VoIP stations; b) APSD groups power-save traffic around Beacon transmissions; and c) APSD requires stations to stay awake to receive power-save traffic from other stations. Thus the needs exists for a single, consistent Simple Power-Save method for VoIP stations that does not require a fast Beacon rate, does not group power-save transmissions around beacon transmissions and does not require stations to stay awake to receive power-save traffic from other stations.